He said mortgages were a good investment, not houses. In terms of EV they're pretty high up the list, especially compared to the alternatives (buying the house outright or renting).

Nothing always goes up, but over time real estate does pretty well. Plus the tax deduction on the interest rate, and the fact that the APR is lower than what you could get with the money if invested well in other stuff make mortgages so well-worth it that even people who could buy outright almost always take them.